This invention relates to a seismic data processing method whereby thickness and velocity characteristics of a subterranean layer can be determined.
It is frequently desirable in seismic exploration for oil and gas to determine the wave velocity characteristics and thickness of a subterranean layer of interest. Velocity characteristics of a layer are valuable to the seismologist in identifying the material which makes up the layer, and the layer thickness provides valuable information as to necessary drilling depths. Conventional reflection and refraction techniques can be used to successfully calculate velocity and thickness of a subterranean layer, however, only if velocity increases continuously with depth. As to techniques utilizing refraction, a seismic wave is transmitted to an interface between layers, where the wave must be refracted so as to travel along the interface and return to the surface of the earth where it is detected. Such a refraction phenomenon cannot occur in the case of a high velocity layer overlying a low velocity layer, since the wave is refracted in the wrong direction (toward the normal rather than away from the normal, wherein the normal is a line perpendicular to the interface and passing through the point at which the wave path intersects the interface). With respect to techniques utilizing reflection, reflections from both boundaries of a layer are required to determine thickness and velocity. Accuracy of thickness and velocity determinations utilizing reflection techniques depend to a large degree on the direction in which the wave is refracted at the upper boundary. More accurate results are obtained where the downward traveling wave is refracted away from the normal by the upper boundary. Where a high velocity layer overlies a low velocity layer, the upper boundary of the low velocity layer will refract a downward traveling wave toward the normal rather than away from the normal. Thus, reflection techniques give inaccurate, and sometimes unuseable, thickness and velocity determinations in this situation.
The above described situation in which a low velocity layer of interest lies under a high velocity layer is often found in frigid regions such as the Arctic. In such regions, an upper layer of the earth is typically frozen year around. This frozen layer is referred to as permafrost. The layer of the earth immediately beneath the permafrost layer is unfrozen, and thus transmits seismic waves therethrough at a lower velocity than the permafrost layer.